In helical-scan VTR's, the recording head produces tracks on the tape which are contiguous, i.e., in which the tracks are immediately adjacent each other or in which there is an intervening gap. An intervening gap, termed a guardband, is used for providing isolation between tracks on playback. When the tracks are abutting with no guardband, isolation between tracks on playback may be provided by what is termed azimuth recording. In azimuth recording, the gap of a recording head is oriented at an angle relative to the direction of head scan movement. This angle is different from the angle of the gap by which the adjacent track is recorded. Thus, the recorded track itself may be said to have an azimuth angle.
Normally, the playback head scans along or follows the same path taken by the recording head. Consequently, so long as the tape speed remains constant as between recording and playback, the playback head can be made to scan the recorded track. If the tape speed on playback deviates from the recording speed, the playback head may not follow the recorded track.
When relatively small differences in tape speed are involved, automatic scan tracking (AST) may be used to keep the playback head on the recorded track. During search modes of operation (moviola modes), however, AST cannot be relied upon to maintain tracking. Consequently, the playback head will leave the track to which it is responsive and partially overlie the region adjacent thereto which may be either another recorded track or a guardband. If it is another recorded track, it may be recorded at an azimuth angle to which the playback head is not responsive, in which case the adjacent recorded track is a guardband as to that head and the track to which the head is responsive. The adjacent region may also be another track recorded at the azimuth angle to which the playback head responds. During that time in which the gap of the playback head partially overlies the recorded track to which it is responsive and the adjacent region, the signal produced by the head from the track to which it is responsive is reduced in amplitude. Any signal transduced to the head by that portion of the adjacent region which a part of the transducer gap in the head overlies, will add or sum with that reduced signal from the recorded track. Since these signals are not coherent (i.e., they are not in a known phase relationship) they add to form random signal levels which constitute noise. Consequently, when a playback head only partially overlies its track, the signal becomes weak and may also become noisy because of extraneous signals from the adjacent regions. Thus, the search mode signals are liable to be weak and noisy.
The prior art copes with such search mode problems differently depending on whether analog or digital recorders are utilized. For analog recorders, the recording head is used during the search mode and the noise bar which results therefrom is merely tolerated. For digital recorders, a frame store is used to accumulate addressed picture portions from each track crossed during a single head sweep. Use of a transducer gap having a width substantially equal to the pitch spacing of the tracks to be scanned has been proposed for providing continuous pickup of coherent information from adjacent tracks during a search mode. However, problems arise with this proposal when the gap reaches a mid-point between tracks where signals may be picked up from both tracks or no signal may be picked up from either track depending on whether the gap width is large or small within the allowable tolerance.